(1) Field of the Invention
The present invention relates to a diffractive optical element that can be used in a variety of applications, such as in optical information processing equipment and optical communication equipment. In particular, the invention relates to a diffractive optical element that polarizes light.
(2) Related Art
FIG. 1 is a cross-sectional drawing showing the construction of a conventional diffractive optical element 200.
The arrows in FIG. 1 show the courses taken by light rays. This is also the case in the following drawings.
As shown in FIG. 1, the diffractive optical element 200 is composed of a substrate 201 that is made of transparent plate glass and has a diffractive optical element pattern 203 formed on a main surface 202.
The incident light L0 is monochromatic and has the wavelength λ. When this light strikes the diffractive optical element pattern 203 on the main surface 202 at an angle of 90°, the light will be diffracted as it passes the diffractive optical element pattern 203, producing zero-order diffracted light L1, and positive first-order diffracted light L2 and a negative first-order diffracted light L3 that each form the diffraction angle θ1 with the light L1.
When it is assumed that the refractive index of the substrate 201 is n (where n>1) and the pattern pitch of the diffractive optical element pattern 203 is Λ, the value of the diffraction angle θ1 can be found easily according to Equation 1 below.θ1=Sin−1{(λ/n)/Λ}  Equation 1
The diffraction angle θ1 found in this way will usually be below the critical angle for total internal reflection by a surface (to be precise, a boundary face of a main surface) of the substrate 201. As a result, the zero-order light L1, the positive first-order light L2, and negative first-order light L3 all pass through the substrate 201 and exit a main surface 204 on an opposite side to the main surface 202.
Polarization is known to occur when the pattern pitch Λ is reduced for the diffractive optical element pattern 203 of this kind of diffractive optical element 200. There are high hopes that this property will enable new kinds of optical elements to be realized.
As Equation 1 clearly shows, reducing the pattern pitch Λ of the diffractive optical element pattern 203 to a value that is equal to or smaller than the wavelength λ of the incident light will result in an increase in the diffraction angle. This gives rise to the problem of the diffracted light being trapped within the substrate 201.
FIG. 2 shows what happens in this case. A diffractive optical element pattern 303 having a pitch that is no greater than the wavelength of the incident light L0 is formed on a main surface 302 of a diffractive optical element 300. Diffraction caused by the diffractive optical element pattern 303 produces positive first-order diffracted light L2 and negative first-order diffracted light L3 that each form a larger diffraction angle θ2 than the diffraction angle θ1 in the case shown in FIG. 1. This diffraction angle θ2 satisfies the condition for total internal reflection by main surface 304, so that the diffracted beams are completely reflected back into substrate 301.
Total internal reflection occurs whenever these reflected beams reach a main surface of the substrate 301, so that the diffracted light ends up being trapped within the substrate 301.
The following describes the case shown in FIG. 3 where a diffractive optical element pattern 213 is formed on a main surface 214 on an opposite side of a substrate 210 to a main surface 212 that is incident to the incident light L0. In FIG. 3, the pattern pitch Λ of the diffractive optical element pattern 213 is greater than the wavelength λ of the light L0. In this case, diffraction by the diffractive optical element pattern 213 produces zero-order diffracted light L1, as well positive first-order diffracted light L2 and negative first-order diffracted light L3 that each form a diffraction angle θ3 with the light L1. This diffraction angle θ3 can be found using Equation 2 below.θ3=Sin−1{(λ/Λ)}  Equation 2
As shown in FIG. 3, the zero-order diffracted light L1, the positive first-order diffracted light L2 and the negative first-order diffracted light L3 each pass through the main surface 214 and out of the substrate 211.
However, when a diffractive optical element pattern is formed in this way, there is still the problem of the diffracted light being trapped in the substrate when the pattern pitch Λ of the diffractive optical element pattern is smaller than the wavelength λ of the incident light.
One example of this case is a diffractive optical element 310 shown in FIG. 4. Diffraction occurs for the incident light L0 that strikes diffractive optical element pattern 313 formed on a main surface 314 of a substrate 311 to produce the zero-order diffracted light L1, the positive first-order diffracted light L2 and the negative first-order diffracted light L3. While the zero-order diffracted light L1 exits the main surface 314, the positive first-order diffracted light L2 and the negative first-order diffracted light L3 do not satisfy the condition for transmittive diffraction, and so are reflected back at a diffraction angle θ4 that is found by Equation 3 below.θ4=Sin−1{(λ/n)/Λ}  Equation 3
These diffracted beams are hereafter subjected to total internal reflection by the main surfaces 312 and 314 and so end up being trapped within the substrate 310.
The above problem means that even if a diffractive optical element pattern is capable of polarizing light, the diffracted beams produced by the diffractive optical element pattern will not exit the substrate. This greatly limits the potential of such substrates as optical elements.
An optical pickup provided in a magneto-optical (MO) disk device reads the information stored on an MO disk by shining a laser beam at an information recording surface of the disk and splitting the light reflected off this surface using a polarizing beam splitter (hereinafter, “PBS”) in the form of a prism. While doing so, the pickup also obtains servo signals, such as the focus error signal and tracking error signal. A prism-shaped PBS is a relatively large component, and so makes miniaturization of the optical pickup difficult.